It was replaced by better link/transclusion markup. The migration scripts converted all old content from 1.5 to the new syntax of 1.6, including replacing usage of ImageLink macro by builtin new syntax. You can still get it from MoinMoin:MacroMarket if you insist on using it. If you used MoinMoin 1.6.0 and your users have added ImageLink macro calls to the content of your wiki, please use full text search to locate them and replace them as you see below: